In the Audience growth section, you can see how many Twitter users are following you, as well as how many new followers you have gained during the selected time period. This number can be positive (if total number of your followers rose) or negative (if total number of followers declined).

There are two available graphs you can switch between - the one showing variation of follower count, i.e. new followers or lost followers, and the second one showing total number of followers over time.